Yield properties and Hall-Petch relationship of the Ti-6Al-4V alloy with an isometric structure and a dual structure

(HY-industry technical centre)

1 Introduction

  • Ti-6Al-4V alloy is one of the most widely used α+β titanium alloys. According to reports, its output exceeds 50% of the world’s titanium production. By changing the processing technology, different types of structure can be obtained in Ti-6Al-4V alloy, including lamellar structure, martensite structure, equiaxed structure and bimorphic structure, etc. Its good combination of strength and plasticity is widely used in production practice.

  • Since the yield strength of the material determines its fatigue performance (especially the low-cycle fatigue performance), the yield strength of the equiaxed structure and bimorphic structure becomes one of the most important mechanical properties. However, the nature of the above-mentioned organizational yielding behavior has not been well understood.

  • In general, the yield characteristics of materials can be studied by two methods: one is direct research, such as in-situ stretching under a scanning electron microscope (SEM) or a transmission electron microscope (TEM); or by an indirect method, That is, the Hall-Petch relationship obtained on the basis of extensive grain size statistics. The in-situ method relies on some special equipment and can only provide information within a limited range, which may not be enough to draw a general conclusion. On the other hand, the relationship between grain size and yield strength can provide general information for the entire microstructure. The research on the Hall-Petch relationship of the titanium alloy with close-packed hexagonal (hcp) structure is still very limited, and the yield strength of the isoaxial structure and the dual structure of the Ti-6Al-4V alloy with different grain sizes has not been compared. the study. In this study, the Hall-Petch parameters of the isometric and bimorphic tissues were determined by tensile experiments, and the essence of yield behavior of different tissues was discussed on this basis.

2 Experimental ideas

  • A cylindrical sample of Φ8×12mm cut from the blank was solution treated at 1050°C for 1h followed by water quenching to obtain a full martensite structure. These samples were then uniaxially compressed at different strain rates (10-4-10-2s-1) at different temperatures (600°C to 950°C) until the true strain was 0.8. After the end of hot compression, in order to obtain a fully equiaxed structure, the sample was slowly cooled to 600°C at 10°C/min, followed by water quenching. On the other hand, in order to obtain the double-state structure, the samples are kept at 850°C or 950°C for different time (5-2400s) after hot compression to obtain grains of different sizes. These two-state structures obtained by holding at 850°C or 950°C Respectively named as BM850 and BM950 dual-state organization.

  • After obtaining the tissues required for the study (full-equiaxed tissues and BM850 and BM950 dual-state tissues), the sample was cut into two parts along the diameter of the cylinder and a section parallel to the compression axis was obtained. Then take the center part of the sample for polishing and standard gold image processing. Scanning electron microscope backscatter (BSE) mode and electron backscatter diffraction (EBSD) analysis were used to characterize different tissues. The tensile test was performed at room temperature with a strain rate of 8.3×10-4s-1 on a Shimadzu AG-X type stretching machine. The microhardness test was carried out on a Hysitron TI950 instrument. A Berkovich diamond probe with a radius of 100 nm was selected to be loaded with 500 μN for 10 s. A 10×10 grid was taken in the range of 3030 μm2 to measure the microhardness.

3 Graphic guide

(a) Schematic diagram of the thermomechanical processing procedure for obtaining the isometric structure; (b) Schematic diagram of the thermomechanical processing procedure for obtaining the two-state structure; (c) Colorization of the inverse pole figure of the isometric structure with an average alpha grain size of 0.3 μm Figure; (d) Coloring diagram of the inverse polar structure of the isometric structure with an average α grain size of 2.1μm; (e) Coloring diagram of the inverse polar structure of the isometric structure with an average grain size of α17.0μm; (f ) Backscattered image of the BM850 dual-state structure with a primary α grain average size of 0.6 μm; (g) Backscattered image of a BM850 double-state tissue with a primary α grain size of 1.9 μm; (h) Average primary α-grain size Backscattered image of BM850 dual-state structure with a size of 14.9 μm; (i) Back-scattered image of BM950 double-state structure with an average α grain size of 1.2 μm; (j) BM950 with an average size of 2.7 μm α Backscattered image of a double-state structure; (k) Backscattered image of a BM950 double-state structure with a primary α grain average size of 12.0 μm.

Thermal compression stage in a is carried out at different temperatures and at different strain rates, and finally an equiaxed structure with different grain sizes is obtained. Through the comparison of cde, it can be found that the deformation temperature and strain rate in the deformation stage can affect the grain size: increasing the deformation temperature and decreasing the strain rate will increase the grain size. In Fig. 1b, according to the different holding temperature after the end of thermal compression, two-state structures with different characteristics (BM850 and BM950) are obtained. Through the comparison and analysis of f to h (or i to k) in Fig. 1, it is found that by controlling the temperature during the deformation stage, the strain rate and the holding temperature after the end of hot compression, a dual-state structure with different grain sizes can be obtained. Through the comparison of hk, it can be clearly observed that the BM850 and BM950 have different characteristics.

(a) Tensile curve of fully equiaxed structure with different grain sizes; (b) Tensile curve of BM850 double state structure with different grain sizes; (c) Tensile curve of BM950 double state structure with different grain sizes ; (d) Hall-Petch relationship fitting curve of three organizations.
Observation of abc shows that for both the equiaxed structure and the BM850BM950 binary structure, the yield strength of the two increases as the size of the primary α grain decreases. For further quantitative analysis, the function of fitting the yield squared strength to the negative square root of the primary alpha grain size is shown. The results indicate that the curve fit between the fully equiaxed structure (R2=0.94) and the BM850 dual-state structure (R2=0.99) is perfect. Hall -Petch parameters are 230MPa·μm1/2 and 232MPa·μm1/2, which shows that there is a traditional Hall-Petch relationship between the two structures, and it is found that when the initial α grain size is close, the yield strength of the fully equiaxed structure is high For BM850 dual-state organization. However, it was found that the point of the BM950 dual-state tissue deviated from the Hall-Petch function line (R2=0.84), and further research is needed.

(a) BM850 dual-state tissue backscatter BSE image for nano-indentation; (b) BM850 dual-state tissue hardness distribution of primary α crystal and β-transformed tissue; (c) 1% plasticity occurs in BM850 dual-state tissue Strain distribution of primary α-crystal and β-transformed tissue during strain; (d) Nano-indentation points on BSE image of BM950 dual-state tissue backscatter; (e) Hardness of primary α-crystalline and β-transformed tissue in BM950 dual-state tissue Distribution; (f) The strain distribution of the primary α-crystal and β-transformed tissue when 1% plastic strain occurs in the BM950 dual-state structure.
Comparing b and e of Fig. 3, it is found that in the process of obtaining the two-state structure, with the increase of the holding temperature after the end of hot compression, the solid solution strengthening effect of Al element is enhanced, and the hardness of β-transformed structure increases obviously, but the There is no change in hardness.

In the comparison of Figure 3c and f, it can be seen that the strain distribution of the BM850 and BM950 dual-state microstructures changes significantly during plastic deformation. This is a perfect fit with the microhardness distribution in Figure 3 b and e. Because the hardness of the β-transformed structure is lower in the BM850 dual-state structure, it undergoes more strain than the primary α crystal during plastic deformation. As the hardness of the β-transformed tissue increases, the strain distribution of the primary α crystal and the β-transformed tissue in the BM950 is more uniform. In addition, in the plastic deformation, since the volume fraction of the β-transformed structure in the BM950 dual-state structure reaches 75%, a matrix is ​​formed, and the primary α crystal nested in the matrix is ​​similar to the particles in particle strengthening. An alloy similar to particle dispersion strengthening is formed. The increase in the hardness and volume fraction of the β-transformed tissue in BM950 resulted in a lower Hall-Petch fit of BM950.

4 Summary

In summary, this paper systematically studied the Hall-petch relationship between the fully equiaxed structure of Ti-6Al-4V alloy and the BM850 and BM950 dual-state structure, and on this basis, discussed the essence of the yield behavior in the three organizations . According to the differences in the ability of the various grain boundaries and interfaces in the microstructure to resist slip transfer, different yield behaviors under different structures and grain sizes can be explained from this angle. The Hall-Petch coefficient was found to be close in the isometric structure and the BM850 double-state structure, which shows that the α/α grain boundary in the isometric structure and the αp/αp grain boundary in the BM850 double-state structure play the same role in the yield behavior . In the BM950 bimorphic tissue, αp plays a role similar to particle strengthening, so the Hall-Petch fit is low.

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